The first line of "if everything happens that can't be done" warns us that we're not going to be in the world we are accustomed to. In the world of this poem, created for just a short 45 lines, everything that can't be done not only can happen, but is happening. This allows even the "stupidest teacher" to discover wonderful things about the world, and we, the readers, can discover them, too. If only we can shake up our traditional notions of language while we're at it.
This poem takes place in a totally different world, because in the world of the poem, anything is possible, and that's not true of our own world.
This poem takes place in the normal world, but the speaker's love transforms it into a space where everything is possible.